1. Field of the Invention
The present invention relates to a magnetic field generating device for applying a bias magnetic field to a recording layer of a magnetooptical recording medium in the magnetooptical recording method.
2. Related Background Art
FIG. 1 illustrates a conventional bias magnetic field generating device employed in the magnetooptical recording method of light modulation type. The bias magnetic field generating device 52 is provided on a magnetooptical recording medium, or a magnetooptical disk 50, so as to generate a substantially uniform magnetic field over the entire information recording area in which an optical head 51 is moved under tracking control. A bias field generating power source 53 supplies current to a coil wound on a core of the bias field generating device 52, and the magnetooptical disk 50 is rotated by a spindle motor 55.
In such magnetooptical recording of a light modulation method, record bits (portions of inverted magnetization) are formed by the intensity change of a light beam from the optical head, and the external field from the bias field generating device is used as a supplement. The recording, reproduction and erasure of information are achieved by intensity change of the light beam. Thus the light beam from the optical head is modulated according to the recording signal.
In such structure, the bias field generating device and the power source therefor are inevitably bulky and heavy since the bias magnetic field has to be generated for the entire information recording area 54. Also because of the difficulty of application of a uniform bias field over the entire information recording area 54, the record pits formed in the magnetooptical disk at recording become uneven in size and shape, eventually giving rise to errors in information reading. Furthermore, because of the significant electric power required for generating the bias magnetic field, the bias field generating device 52 causes temperature increase of the magnetooptical disk 50, thereby affecting the conditions of information recording and eventually leading to errors.
For this reason there is also proposed a bias field generating device for applying a bias field only to an area currently subjected to magnetooptical recording by the optical head, and this is called the magnetooptical recording of a magnetic field modulation method. FIG. 2 illustrates such recording, wherein a bias field generating device 62 is linked, by a support member 66, to an optical head 61 positioned across a magnetooptical disk 60, and said device 62 and optical head 61 are mutually so positioned that the light beam 67 for information recording and reproduction from the optical head 61 substantially coincides with the center of the bias field generated by the device 62. Consequently the bias field generating device 62 can follow the movement of the optical head 61 when it is moved in the information recording area 64 under tracking control.
In such magnetooptical recording of the magnetic field modulation method, the recording signal is introduced into the external magnetic field of the bias field generating device. At the information recording, the light beam from the optical head has a constant intensity and is used as a supplement. Thus the external field from the bias field generating device is modulated according to the recording signal.
In such magnetooptical recording, the support member 66 linking the bias field generating device 62 and the optical head 61 is provided, in a part A, with a spring of desired properties, and said bias field generating device 62 is made to float with a small spacing from the surface of the magnetooptical disk 60, utilizing the air flow generated by the rotation of the magnetooptical disk 60 on said surface.
Thus the bias field generating device shown in FIG. 2 can avoid the drawbacks of the bulk, weight and large power consumption associated with the device generating the magnetic field over the entire recording area as shown in FIG. 1, and can therefore eliminate the causes of errors.
However the bias field generating device shown in FIG. 2 is still associated with the following drawbacks. In such a device, since the generated bias field has only a narrow uniform area, so that the light beam 67 from the optical head 61 has to be exactly aligned with the center of the magnetic field generated by the device 62. For this reason there are required highly precise designing, manufacture and alignment, which lead to increased costs.
Also, the bias field generating device can follow the movement of the optical head through the support member 66, when said optical head is moved by a large amount under tracking control, but, in the course of scanning with the light beam, there is required correction control for the vibration of the light beam 67 resulting from vibration of an optical head actuator or for the aberration between the light beam 67 and the center of the bias field resulting from the vibration of the generating device 62. Particularly, in the tracking control of the light beam 67 by a fine tracking control actuator in the optical head 61, the correction control is indispensable for the positional aberration of said bias field generating device 62.
It is therefore conceivable to maintain the uniform area of the bias magnetic field of the generating device at a certain size, thereby enlarging the tolerance of said positional aberration and practically dispensing with said correction control. However, in the above-mentioned bias field generating device, the cylindrical coil employed therein requires a considerable number of turns for generating a required magnetic field, and the magnetic field is difficult to concentrate as the coil requires a certain distance in a direction perpendicular to the magnetooptical disk. Therefore if the magnetic pole is made larger for enlarging the uniform area of the necessary magnetic field, the high frequency drive becomes difficult due to the increase in inductance, and the bias field generating device inevitably becomes bulky and heavy.